Dark matter detector finds tiny cracks instead

A CHANCE discovery by cosmologists searching for dark matter – the elusive stuff thought to make up most of the mass in the universe – could lead to the development of an ultra-sensitive probe that can spot cracks in materials opening up, even when they’re only a few atoms long.

Dark matter detectors at the Cryogenic Rare Event Search using Superconducting Thermometers (CRESST) underground laboratory at Gran Sasso, Italy, were being put through preliminary tests in 1999 when they began recording thousands of times more signals than expected. “We were very disturbed,” says Leo Stodolsky, a team member at the Max Planck Institute for Physics in Munich, Germany. “The set-up is so sensitive that touching it with bare hands can cause problems. At first, we could only think some idiot had handled it.”

The detectors, each of which consists of a cryogenically cooled sapphire crystal and a superconducting thermometer, are designed to measure the heat that would be given off if dark matter collided with particles in the crystal. Dark matter striking the crystal lattice would strip electrons from the crystal’s atoms and generate vibrations that would warm the crystal by a tiny amount.

The researchers finally traced the signals to micro-fractures in the detectors, caused by tight clamping. They have now established that the signals were created by energy produced as just a few hundred atomic bonds began to break.

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This is the first time the energy released by such small-scale fracturing has been measured directly, says Stodolsky. “This suggests a new technology that could go down to the atomic level.”

Stodolsky and his colleagues hope to use the technique, to be reported in Physics Letters A, to build detectors for examining and monitoring materials at the atomic scale. Such detectors could also help researchers developing new materials by testing them for microscopic strains.

The micro-fractures could also give us an insight into the mechanics of earthquakes, says Stodolsky. The team’s findings show that the bursts produced by the micro-fractures – early large signals followed by many smaller ones – are similar to those observed in earthquakes.

“It is very interesting to see phenomena on a truly microscopic scale that we also observe in earthquakes and climate,” says Armin Bunde at the University of Giessen, Germany.